In the process of xerography, a light image corresponding to the original to be copied is typically recorded in the form of a latent electrostatic image upon a photoconductive member. This latent image is developed, that is to say made visible, by the application of a pigmented thermoplastic resin, commonly referred to as toner. The visible image is thereafter transferred from the photoconductive member onto a copy sheet, such as, for example, paper. The copy sheet is subsequently passed through a fusing apparatus which affixes the image onto the copy sheet and is later discharged from the machine as a final copy.
One approach to fixing the toner particles onto the copy sheet has been to pass the copy sheet with toner images thereon, through a fusing nip formed by a heated fuser roll and a backup roll, the copy sheet being so oriented that the side thereof bearing the toner image contacts the heated fuser roll. As it passes through the nip, the copy sheet is simultaneously pressed and heated so that the toner becomes softened and firmly attached to the copy sheet.
As compared to other thermal fusing techniques, the heated roll type is considered more efficient as the time required for fusing the toner image onto the copy sheet is substantially reduced by providing for the simultaneous heating and direct compression of the toner image. Further, the size of the copying apparatus can be minimized due to the reduced space required for heated roll type fusing assemblies.
One of the disadvantages of such a fusing arrangement, however, is the relatively narrow surface temperature range that must be maintained by the heated fuser roll in order to properly fuse the toner image onto the copy sheet. If the surface temperature of the heated fuser roll is allowed to fall below this optimal range, a phenomenon referred to in the printing art as "offset" often results, i.e., wherein toner adheres to the roller surface and is transferred to the next copy sheet. Similarly, where the surface temperature of the heated fuser roll is higher than the optimal fusing temperature, the toner becomes over-fused and adheres to the roller surface simultaneously with fusion onto the copy sheet so that the adhered toner is transferred to the next copy sheet. Overheating may additionally result in paper jamming, as the copy sheet will tend to follow the heated fuser roll, rather than continuing along the intended paper path beyond the fuser station.
It is also essential in such a toner fixing arrangement so as to insure proper fusing of the toner image, that adequate pressure be applied between the heated fuser roll and the backup roll while the copy sheet is disposed therebetween. Further, since the fusing of the toner image is effected by a single application of heat and pressure, it is important that the heated fuser roll and backup roll be positioned axially parallel to each other so that there is minimal variance in the degree of fusion.
Known techniques of fuser roll design indicate the desirability of (1) providing a heat source internal of the heated fuser roll to minimize heat loss, (2) providing a deformable surface on the heated fuser roll to minimize the sticking of fused copies thereto, and (3) maximizing the "footprint" or impression made by the backup roll into the deformable surface of the heated fuser roll to maximize time for heat transfer. It has been recognized, however, that in many instances, the various design techniques are mutually conflicting. For example, maximization of the "footprint" increases the resident time of the copy sheet against the heated fuser roll surface. Consequently, fusing may then be achieved at a reduced operating temperature with an accompanying improvement in energy efficiency. Obviously though, the force or stresses applied by the backup roll as it contacts the heated fuser roll must be increased to produce this "larger" footprint, a consequence that may not be so desirable as the durability of the rollers can be measured as a function of the mechanical stresses thereon.
Similarly, the deformable surface material desirable for the heated fuser roll so as to provide the best separation of copy therefrom, conflicts with the criteria necessary to achieve the best heat transfer through the heated fuser roll surface from an internal heat source. Materials considered best suited to providing the deformable surface of the heated fuser roll, such as, for example, silicone polymers and elastomers, have only fair heat conducting properties. Thus, to obtain an efficient heat conducting path, it is necessary to limit the thickness of the deformable surface. Prevention of sticking on the other hand, is enhanced by a thick deformable surface layer of these materials. Further, a relatively thin deformable surface layer limits the total size of the footprint and also increases the force and attendant stresses required to develop a footprint of any given size.
Accordingly, it is a principal object of this invention to provide an improved xerographic toner fixing apparatus.
It is another object of this invention to provide a toner fixing apparatus capable of fusing a toner image onto a copy sheet at a reduced temperature and applied pressure and applied load.
It is a further object of this invention to provide a toner fixing apparatus wherein fusing may be achieved within a broader operating temperature and pressure range than heretofore.
Still another object of this invention is to provide a toner fixing apparatus which is easily fabricated.